Broadly, the invention relates to a valve for controlling fluid flow in a wellbore. More specifically, the invention is directed to a valve assembly useful for controlling liquid flow between a well casing and a tubing string positioned in the casing.
In the production of oil and gas various techniques are used to enhance recovery of the petroleum product from earth formations having low permeability. Hydraulic fracturing is an example of such a technique. This procedure involves pumping liquids under high pressure down the well casing and into the producing formation, which fractures the formation outwardly from the well casing. The fractures thus provide a larger surface area in the formation, to enable better drainage of the oil or gas into the well casing.
In a typical fracturing operation the liquid used to treat the formation is pumped down a tubing string suspended inside of the well casing. Residual fluids, such as drilling mud, salt water, and the like, remain in the casing after the well is drilled and the casing is cemented into the borehole. When the tubing string is lowered into the casing these residual fluids become trapped in the annulus between the tubing string and the well casing. During a fracturing operation the fluids in the casing annulus can backflow into the tubing string and mix with the fracturing (treating) fluid. This is an undesirable situation, in that the residual fluids can contaminate the fracturing fluid and destroy the intended purpose of the treatment. The valve assembly of this invention provides a tool which allows fluid to be pumped down the tubing string, and at the same time prevents fluid in the casing annulus from backflowing into the tubing string.